The present invention relates to a benzo[a]naphthacene compound having .alpha.-glucosidase inhibiting activity. The compound of the present invention is therefore useful as human and animal medicament for the treatment of conditions in which it is desirable to suppress .alpha.-glucosidase action, or to inhibit the increase of blood glucose level; such conditions include, for example diabetes, prediabetes, obesity, and adiposity.
Recently, Japanese Kokai 2-83351, published Mar. 23, 1990, reported the isolation of a novel .alpha.-glucosidase inhibitor from fermentation broth of Actinomycetes Strain MH193-16F4. The inhibitor, benanomicin C, has the following structure formula ##STR2##
Although benanomicin C also possesses a benzo[a]naphthacene ring nucleus, its substituents differ substantially from those of the compound of the present invention. Furthermore, the compound of the present invention exhibits unexpectedly high .alpha.-glucosidase inhibitory activity compared to benanomicin C.